Benzoyl peroxide is a non-toxic, colorless, odorless and tasteless crystalline solid with a molecular weight of 242.22 and a melting point of between about 103.degree. to 106.degree. C. Pure (98% active) benzoyl peroxide crystals are commercially available and are extremely flammable and shock sensitive. Accordingly, pure benzoyl peroxide crystals are not normally used in the preparation of cosmetic or pharmaceutical products. Benzoyl peroxide is also commercially available as a 75% crystalline solid with 25% water. These wet, crystalline solids are less flammable and less shock sensitive than the pure, dry crystals and therefore allow safe shipping of the otherwise flammable crystals.
The crystalline powder is gritty and extremely hard and requires milling for several hours in water through high shear mills to prepare a paste having benzoyl peroxide crystals that are sufficiently fine to be of acceptable texture for preparing products for topical use. The aqueous paste prepared from this tedious, expensive and time consuming process can then be used to prepare cosmetic emulsions, containing fine particles of benzoyl peroxide in suspension to form creams, lotions or gels. The milling process must be carried out at low temperature to maintain the stability of the benzoyl peroxide, which starts decomposing at temperatures over 40.degree. C.
Although benzoyl peroxide is soluble in some industrial solvents, such as aromatic solvents, including toluene, methyl-ethyl ketone and benzene, these industrial solvents are highly flammable and are generally too irritating for human use. Therefore preparations containing these industrial solvents are unusable in the over-the-counter or prescription drug preparations. Although acetone is used to a limited degree in some topical foundations, the use of acetone is greatly limited by the fact that acetone is extremely drying to skin and is flammable.
It would be desirable to provide benzoyl peroxide compositions which can be easily and economically prepared, which have a smooth texture appropriate for cosmetic products, and which are non-irritating when placed into contact with skin or other tissue.